Tokyo (CNN) -- At least six people were killed when a magnitude-6.4 earthquake struck Japan Tuesday morning, a local fire department said.

The six were killed when a landslide triggered by the quake buried three homes in Iwaki, the city's fire department said. Three people were rescued and hospitalized, and fire officials were working to rescue an unknown number of others believed to be trapped, the department said.

The quake struck at about 8:08 a.m. Tuesday (7:08 p.m. Monday ET), according to the U.S. Geological Survey. It had a depth of about 13 kilometers (8 miles) and was centered about 77 miles east-southeast of Tokyo.

Monday night, one person was killed in Iwaki and several others were trapped when a powerful 6.6-magnitude earthquake triggered landslides there, the fire department said. It happened exactly one month after the country's devastating 9.0-magnitude quake and tsunami.

The earlier quake was centered about 100 miles (164 kilometers) northeast of Tokyo and about 30 miles (50 kilometers) southwest of the crippled Fukushima Daiichi nuclear plant, according to the U.S. Geological Survey.

The landslides in Iwaki buried three houses. Police in Fukushima Prefecture initially reported that four people were trapped. The Iwaki Fire Department later said more than four people were trapped, but the exact number was unclear.

Authorities were trying to rescue them. Their condition was not immediately known, police said.

Workers at Fukushima Daiichi nuclear plant, which has suffered cooling problems and radiation leaks since the March 11 earthquake and tsunami, evacuated briefly but soon returned to resume their efforts to cool the troubled facility.

Prime Minister Naoto Kan delayed a scheduled news conference as a result of the earthquake, his office said. The government wants to handle the immediate emergency before holding a news conference, the office said.